The invention relates to a semiconductor device comprising a semiconductor body including, in succession, a first and a second semiconductor region of a first conductivity type having, respectively, a high and a low doping concentration, and including a third semiconductor region of a second conductivity type, contrary to the first conductivity type, and having a high doping concentration, at least in the second semiconductor region, the service life of the charge carriers being reduced, and the first and the third semiconductor region being provided with, respectively, a first and a second connection conductor.
Such a device is used as a switching diode, for example in a power supply, and also operates very well at high frequencies, while, in addition, the forward voltage drop is low.
Such a diode is disclosed in United States patent specification U.S. Pat. No. 4,594,602, published on Jun. 10, 1986. Said document describes a diode having an n++ substrate on which a thick n+ epitaxial layer and a thinner n epitaxial layer are situated, which form a first and a second semiconductor region of the diode. In the n epitaxial layer there is a local p+ diffusion which forms a third semiconductor region of the diode. This region is provided with a connection conductor as is, via the substrate, the first semiconductor region.
A drawback of the known device resides in that it is only extremely suitable for use at a high voltage. In accordance with the above-mentioned patent specification, a Schottky diode can suitably be used as an alternative for low-voltage applications. Such a diode is comparatively expensive, however, as a result of the specific production steps required to manufacture a Schottky diode.
Therefore, it is an object of the invention to provide a diode which, like a Schottky diode, can suitably be used for low-voltage applications, and which is inexpensive and, otherwise, has favorable properties, such as a high switching speed and a low forward voltage drop.
To achieve this, a device of the type mentioned in the opening paragraph is characterized in accordance with the invention in that the thickness of the second semiconductor region is reduced to below 20 xcexcm, and the service life of the charge carriers in the second semiconductor region is reduced to below 20 nsec. By virtue of such a substantial reduction of the service life of the charge carriers in the second semiconductor region and such a small thickness of the second semiconductor region, the diode in accordance with the invention, just like a Schottky diode, is capable of switching very rapidly. In addition, a diode in accordance with the invention operates at relatively low reverse voltages of, for example, 10 to 100 V. Very surprisingly, it has been found that such a diode also has a very low forward voltage drop, which is very important. As a result, the invention is based on the following, surprising recognition. In a pn diode having a relatively thick base layer (i.e. the second semiconductor region), the reduction of the service life of the charge carriers leads to an increase of the forward voltage drop, while, on the contrary, in a device in accordance with the invention, said reduction of the service life of the charge carriers leads to a reduction of the forward voltage drop. This can be attributed to the fact that a reduced service life of charge carriers gives rise to an increased recombination current in the base layer, while the resistance in the neutral part of the diode does not increase appreciably by the reduction of the service life of the charge carriers. This is in contrast to diodes wherein the base layer is relatively thick. In this respect, it is also important that it is the neutral part of the diode which is responsible for the series resistance of the diode. The forward voltage drop of a device in accordance with the invention thus comes close to a value which is customarily found in a Schottky diode. As a result, a device in accordance with the invention is an attractive alternative to a Schottky diode.
Particularly favorable results are obtained if the thickness of the second semiconductor region is less than 5 xcexcm, and the service life of the charge carriers in that region is less than 5 nsec. A lower limit of the thickness is approximately 0.5 xcexcm because at an even smaller thickness, the maximally attainable reverse voltage becomes impractically low, while a practical lower value of the service life is approximately 1 nsec. Very attractive results are obtained within a range which is approximately a factor of 2 to 5 larger than said lower limits.
Platinum atoms prove to be very suitable for obtaining said short service life of the charge carriers in the second semiconductor region. Surprisingly, it has been found that by using these atoms, are not only the desired short service life can be obtained but also the characteristics of the device are not adversely influenced. This is exceptional, because both the second and the third semiconductor region of a device in accordance with the invention are very thin. Another attractive way of achieving the intended low service life of the charge carriers consists in exposing the semiconductor body to radiation with electrons or protons.
A preferred application of a device in accordance with the invention relates to the use thereof as a fast rectifier diode in (the secondary side of) a so-called SMPS (=Switch Mode Power Supply) unit. A so-called flyback converter and a so-called buck converter are examples of such a SMPS unit. This is a very attractive application because as a result of the ongoing miniaturization, the switching frequencies in a SMPS unit are very high so that rapidly switching diodes with a minimal power dissipation and hence a minimal forward voltage drop are necessary. Owing to the very large market for such diodes having a high switching speed, the price of a device in accordance with the invention is an important advantage in this case.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment described hereinafter.